Pyruvate kinase (PK) is a critical metabolic enzyme operating at the ultimate step in glycolysis where it catalyzes the transfer of a phosphate group from phosphoenolpyruvate to adenosine diphophate (ADP), yielding one molecule of pyruvate and one molecule of adenosine triphosphate (ATP). In humans there are two pyruvate kinase genes and each produces two distinct gene products by alternative splicing. The L gene produces two different mRNAs that differ only in the first exon to produce the L (liver specific) and R (red blood cell) specific isozymes. Splicing of a single exon within the M gene produces the M1 isozyme that is found in most adult tissues and the M2 isozyme that is present in fetal tissues and is found to be re-expressed in tumors. Therefore, after embryonic development, adult tissues switch to either express PK-M1 or the tissue specific L or R isozymes. However, in all tumors or cell lines of cancer lineage (including those typically expressing either the L or R isozymes), PK gene expression reverts entirely to the M2 isoform.
PK is a tetrameric enzyme composed of four identical monomers that form a dimer of dimers in the final tetrameric structure. In humans, the M2, L, and R isozymes are activated by fructose-1,6-bis phosphate (FBP) that binds to a flexible loop region at the interface of the two dimers. Activation of PK shifts the enzyme to a state showing high affinity for phosphoenolpyruvate (PEP). In contrast, the M1 isoform is not regulated by FBP and displays only high affinity PEP binding similar to the activated state of PK.
Tumor cells undergo a metabolic transformation that is required to supply the biochemical precursors necessary for rapid cell growth and proliferation.
Various phosphotyrosine peptides can bind to PK-M2 near the activation loop that results in the removal of FBP from the enzyme which effectively down-regulates PK-M2 activity. When PK-M2 is activated, glucose is converted to pyruvate. However, when PK-M2 is inactivated, a build-up of glycolytic intermediates occurs which intermediates can be diverted towards nucleotide and lipid biosynthesis required for cell growth and proliferation.
In addition, PK deficiency is the second most common cause of enzyme-deficient hemolytic anemia, following G6PD deficiency. In patients with PK defiency, a metabolic block is created in the pathway at the level of the deficient enzyme. Intermediate by-products and various glycolytic metabolites proximal to the metabolic block accumulate in the red blood cells, while such cells become depleted of the distal products in the pathway, such as lactate and ATP. The lack of ATP disturbs the cation gradient across the red cell membrane, causing the loss of potassium and water, which causes cell dehydration, contraction, and crenation, and leads to premature destruction of the red blood cell. The survival of patients with severe PK deficiency depends on a compensatory expression of the PK-M2 isozyme, widely distributed in various tissues, including red blood cells, in which the PK-M2 is the R isozyme.
Accordingly, there is a desire for new activators of PK-M2.